The present invention relates to an apparatus and method for determining a condition of road surface. More particularly, the invention relates to an apparatus and method for determining a condition of road surface, which can increase the performance and safety of a vehicle by judging a coefficient of friction occurring between a road and a tire (road friction coefficient) by using rotational information of four tires.
When a vehicle suddenly accelerates or stops on a slippery road, there is a risk that the tires slip and spin. By sudden movements of the steering wheel, there is a possibility that the vehicle skids or spins.
Conventionally, there have been proposed techniques such as an antilock braking system (ABS) for reducing a brake torque acting on wheels to prevent the wheels from being locked before a damping force between tires and a road exceeds the maximum value and the tires are locked, thereby controlling the rotational speed of the wheels at which the maximum damping force can be obtained (Japanese Unexamined Patent Publication Nos. 99757/1985 and 249559/1989 and the like).
For example, in the control of the antilock braking system, a slip ratio is calculated from a judged vehicle speed and a detected wheel speed (rotational speed) and, after that, a braking force is controlled so that the calculated slip ratio coincides with a preset reference slip ratio. In such a manner, the maximum damping force is followed.
In the control of such an ABS or the like, a coefficient xcexc of friction on a road is used. Specifically, according to the coefficient xcexc of friction on a road (road friction coefficient xcexc), that is, whether the road friction coefficient xcexc is high or low, the contents of a control are changed to thereby perform the optimum control.
In the system of Japanese Unexamined Patent Publication No. 99759/1985, vehicle acceleration is calculated from a following wheel at the time of occurrence of a slip, and the road friction coefficient xcexc is judged by using the acceleration.
More specifically, according to the technique disclosed in the publication, when the vehicle acceleration at the time of slip is A and the vehicle weight is W, a driving force F necessary to accelerate the vehicle is calculated as follows.
F=Wxc2x7A/g (where, g denotes acceleration due to gravity) 
On the other hand, the driving force F is determined by a frictional force generated between the driving wheel and the road and can be expressed as F=xcexcxc2x7Wr by using a load Wr applied on the driving wheel and the road friction coefficient xcexc. The road friction coefficient xcexc can be calculated from the above two equation as xcexc=W/Wrxc2x7gxc3x97A.
The road friction coefficient xcexc obtained by the equation is, however, a road friction coefficient xcexc at the time when the vehicle acceleration A is calculated by replacing the acceleration of the following wheel derived by simply differentiating the rotational speed of the following wheel with the vehicle acceleration A. It is not known whether or not the road friction coefficient xcexc is the actual coefficient xcexc of friction between the road and the tire. The probability that it is not the actual road friction coefficient xcexc is overwhelmingly high.
When various vehicle dynamic controls of an ABS and the like are performed on the basis of such a road friction coefficient xcexc, since the controls are not based on the actual road friction coefficient xcexc, it is feared that improper controls are executed. Also in the case of alarming the driver that the road is slippery, when the judged road friction coefficient xcexc is used, an erroneous alarm might be given. Further, the slip ratio at which the maximum damping force is obtained varies according to the road friction coefficient xcexc. Consequently, when a follow-up control is performed on the basis of a fixed slip ratio, it is feared that the braking distance becomes longer or tires are locked.
As a measure, it is necessary to judge the road conditions and vary a reference slip ratio according to the judged road friction coefficient xcexc. There is not, however, a technique for judging the road friction coefficient xcexc with precision.
An object of the invention is to provide an apparatus and method for determining a condition of road surface, which can increase the performance and safety of a vehicle by judging a coefficient of friction occurring between a road and tires by using rotational information of four tires.
In accordance with the present invention, there is provided an apparatus for determining a condition of road surface comprising: rotational speed detecting means for periodically detecting rotational speeds of four tires of a vehicle; first computing means for computing a slip ratio from measurement values of the rotational speed detecting means; second computing means for obtaining a relational formula between the slip ratio and either the acceleration or deceleration of the vehicle; and friction coefficient judging means for judging a coefficient of friction occurring between a road and a tire on the basis of a slope of the relational formula obtained by the second computing means.
In accordance with the present invention, there is also provided a method for determining a condition of road surface comprising the steps of: periodically detecting rotational speeds of four tires of a vehicle; computing a slip ratio from the measured rotational speeds; obtaining a relational formula between the slip ratio and either the acceleration or deceleration of the vehicle; comparing a slope of the relational formula with a preset threshold; and judging a coefficient of friction occurring between a road and a tire from a result of the comparison.
In accordance with the present invention, there is further provided an apparatus for determining a condition of road surface comprising: rotational speed detecting means for periodically detecting rotational speeds of four tires of a vehicle; first computing means for computing either the acceleration or deceleration of the vehicle and a slip ratio of tires from measurement values of the rotational speed detecting means; moving-average calculating means for calculating a moving average from the acceleration or deceleration of the vehicle and the slip ratio in predetermined time; weighted moving-average calculating means for performing moving average on the moving average value obtained by the moving-average calculating means; second computing means for obtaining a relational formula between the acceleration or deceleration of the vehicle and the slip ratio from weighted moving averages obtained by the weighted moving average calculating means; and friction coefficient judging means for judging a coefficient of friction occurring between a road and a tire on the basis of a slope of the relational formula obtained by the second computing means.
In accordance with the present invention, there is still further provided a method for presuming a condition of road surface comprising the steps of: periodically detecting rotational speeds of four tires of a vehicle; computing acceleration or deceleration of the vehicle and a slip ratio of tires from the measured rotational speed; calculating a moving average of the acceleration or deceleration of the vehicle and the slip ratio in predetermined time; performing moving average on the moving average value obtained in the moving average calculating step; obtaining a relational formula between the acceleration or deceleration of the vehicle and the slip ratio from the weighted moving averages obtained by the weighted moving average calculating means; comparing a slope of the relational formula with a preset threshold; and judging a coefficient of friction occurring between a road and a tire from a result of the comparison.